It has long been known to utilize hot exhaust from internal combustion engines as the power supply to absorption refrigeration units or to absorption air conditioning units, i.e., closed NH.sub.3 --H.sub.2 O absorption cycles or closed H.sub.2 O-LiBr absorption cycles. More recently, with increased emphasis on energy conservation, it has additionally been disclosed to apply engine cooling circuit heat to the same purpose.
Sung (U.S. Pat. No. 4,380,909) applies both exhaust gas heat and cooling circuit heat to a single desorber. This has the disadvantage that the desorber must operate at a temperature lower than the temperature of the cooling circuit heat. Since the exhaust gas is at a much higher initial temperature, its extra availability is wasted thereby.
Mori, et al., (U.S. Pat. No. 4,439,999) avoid that limitation by applying the exhaust gas heat to a high temperature desorber, and the cooling circuit heat to a different lower temperature desorber. However, the Mori, et al., disclosure shares the disadvantage of Sung that the cooling circuit heat is normally approximately 110.degree. C. or lower, and that a desorber heated by heat at that temperature is of limited usefulness.
It is also known in the prior art to raise the pressure of exhaust low pressure steam to a higher useful pressure by means of a steam-powered open cycle solution thermocompressor, which uses an aqueous solution of either KOH or NaOH as the absorption medium. Low pressure steam is absorbed into KOH, releasing heat to boil part of the useful steam. High pressure steam is then used to heat and reconcentrate the KOH, boiling the remaining fraction of the useful steam out of the KOH. A pump circulates the KOH solution between the absorption and desorption steps. L. S. Marks describes the basic system in "A Steam-Pressure Transformer", Mechanical Engineering, June 1927, Vol. 49 No. 6, p. 600. A variation which incorporates two desorbers with only the higher temperature one being heated by the high pressure steam is reported by W. H. Sellew in "Solution Cycles", Transactions of the American Institute of Chemical Engineering, Volume XXX, 1933-34, p. 562.
The above-disclosed solution compressors are limited in effectiveness due to the high temperature corrosion limitations of the disclosed absorbent media; the susceptibility to chemical reaction with trace impurities (e.g., CO.sub.2) of the disclosed absorbent media; and the need for high pressure steam to power the solution compressors.
It is known to use an aqueous mixture of alkali metal nitrates in an absorption cycle apparatus for reversibly absorbing and desorbing water vapor: U.S. Pat. Nos. 4,454,724, 4,563,295, and 4,652,270, all assigned to D. C. Erickson.
Most internal combustion engines incorporate an engine cooling circuit. Particularly for reciprocating engines, the heat rejected to the engine cooling system (jacket or cylinder cooling) can approach or exceed the usable content of the engine exhaust gas. Whereas exhaust gas temperature may approach 600.degree. C., cooling circuit temperature is closer to 100.degree. C.
Reciprocating engines are desirable in cogeneration applications due to their high electrical efficiency. However the very low temperature of almost half of their exhaust heat is a severe offsetting disadvantage. In many applications the cooling circuit heat is simply too cold to be useful, and hence a heat rejection system such as a cooling tower or radiator must dissipate it. In other applications, e.g., a hospital, the heat is useful for space or hot water heating, but requires an expensive hydronic or low pressure steam system to distribute it where needed.
What is needed, and one object of this invention, is a means for upgrading the heat from an internal combustion engine cooling circuit into useful pressure steam, e.g., at a pressure of 300 kPa or higher, using only the excess temperature availability in the exhaust gas as the driving force for that upgrading. In other words, it is desired to combine both the low temperature engine cooling heat and the high temperature exhaust gas heat into useful medium temperature heat, in the form of steam, without requiring any mechanical vapor compression apparatus or any significant quantity of mechanical or electrical power input. It is further desired to exhibit a low corrosion rate using low cost materials, of construction, and to avoid contamination by trace impurities.